There is a multi-billion dollar global market for semiconductor and other surface defect management which is growing both in absolute terms and as a percentage of capital equipment investment. For example, in general, there are two factors that determine the economics of a semiconductor fabrication facility at a given utilization level, namely throughput and yield. As complex new technologies such as 300 mm semiconductor wafers, copper interconnects, and reduced feature (circuit) sizes drive the margin of error in fabrication ever lower, new inspection technologies are critical to keep yields high and bottom-line economics attractive. Detection and elimination of chemical contamination and other types of defects is a constant concern for semiconductor manufacturers and equipment suppliers. Contamination can arise from use of processing chemicals, processing equipment, and poor handling techniques. Contaminants can include, for example, metals, carbon, and organic compounds. Other types of defects can result from a wide range of causes, including flaws in the semiconductor crystal, improper processing, improper handling, and defective materials. In addition, many cleaning steps are required in wafer fabrication, such as but not limited to the semiconductor industry. Each step is time consuming and requires expensive chemicals that may require special disposal procedures. Existing methods for monitoring or controlling these processes are expensive and time consuming. As a result, wafers are often cleaned for a longer period of time, using more chemicals than are required.
Continuing with the example of the semiconductor wafer industry, contamination and chemical residue are two major causes of reduced yield in semiconductor fabrication. Wafer cleanliness is becoming even more important as minimum feature sizes shrink below 90 nm, where the thickness of adsorbed layers and organic contaminants are on the same order as the process tolerances of functional films in devices. Contamination, whether organic or metallic, may generate process variation and defects such as poor coverage, vacancies, voids, leaking, shorting, and overburdens. For example, a small amount of metal contamination on the wafer surface may diffuse into the bulk semiconductor and cause the bulk minority carrier lifetime to decrease because the metal contamination may assist in the recombination of electrons and holes in the semiconductor substrate. Reducing contamination and minimizing residues are important factors in increasing yields in semiconductor wafer fabs.
Defect detection and characterization systems, such as in the semiconductor wafer industry, can usually be divided into in-line and off-line systems. “In-line” refers to inspection and measurement that takes place inside the clean room where wafers are processed. “Off-line” refers to analysis that takes place outside of the wafer processing clean room, often in a laboratory or separate clean room that is located some distance from the manufacturing area. In addition, many of these analytical techniques are destructive, which requires either the sacrifice of a production wafer or the use of expensive “monitor” wafers for analysis. In-line inspection and measurement is crucial for rapidly identifying and correcting problems that may occur periodically in these types of manufacturing processes. A typical semiconductor wafer can undergo over 500 individual process steps and require weeks to complete. Each semiconductor wafer can have a finished product value of up to $100,000. Because the number of steps and period of time involved in wafer fabrication are so large that substantial work in process can exist at any point in time. It is critical that process-related defects be found and corrected immediately before a large number (and dollar value) of wafers are affected. Such defects, regardless of the nature of the wafer, semiconductor, IC, or other device, are detrimental to performance and diminish productivity and profitability.
Many types of defects and contamination are not detectable using existing in-line tools, and these are typically detected and analyzed using expensive and time-consuming “off line” techniques (described below) such as Total Reflectance X-ray Fluorescence (TXRF), Vapor Phase Decomposition Inductively Coupled Plasma-Mass Spectrometry (VPD ICP-MS) or Secondary Ion Mass Spectrometry (SIMS). Since these techniques are used off-line (outside of the clean room used to process wafers) and usually occur hours, or even days, after the process step that has caused the contamination, their value is significantly limited.
A brief description of some well known techniques for wafer inspection and chemical contamination detection are presented in Table 1. This list is not in any sense exhaustive as there are a very large number of techniques that are used for some type of semiconductor analysis or characterization or for other surface inspection of other types of materials.
TABLE 1Analytical TechniqueDescriptionIn-line/Off-lineTotal Reflection X-RayX-rays irradiate the wafer within theOff-lineFluorescence (TXRF)critical angle for total external reflectance,causing surface atoms to fluoresce.Automated Optical MicroscopyOptical images are acquired andIn-lineautomatically analyzed for detection oflarge defects.Laser BackscatteringWafer surface is illuminated with laserIn-linespots and the angle and/or polarization ofreflected light is analyzed to detect andclassify particles.Vapor Phase DecompositionWafers “scanned” with a drop of HF that isOff-lineInductively Coupled-Massanalyzed using mass spectrometry.Spectrometry (VPD ICP-MS)Secondary Ion MassIon beam sputters the wafer surfaceOff-lineSpectroscopy (SIMS)creating secondary ions that are analyzedin a mass spectrometer.
Table 2 summarizes some major advantages and disadvantages of each example technique. In general, off-line detection techniques are extremely sensitive to tiny amounts of contamination; but such techniques are slow, expensive, and complex to operate. Some have limited, or no, imaging or surface mapping capability, or are destructive in nature. In-line techniques are much faster, non-destructive, and provide defect mapping, but have limited chemical contamination detection or analysis capability.
TABLE 2Analytical TechniqueAdvantagesDisadvantagesTotal Reflection X-RayVery sensitiveLimited coverageFluorescence (TXRF)Some mapping capabilityUnpatterned wafers onlyNondestructiveAutomated OpticalFastVery limited chemical andMicroscopyRelatively low costparticle detectionDetects a wide range of macrodefects (>50 microns)Imaging of wafer surfaceNon-contact/non-destructiveLaser BackscatteringFastOnly detects particles - noRelatively low costchemistryDetects very small particlesImaging of water surfaceNon-contact/non-destructiveVapor PhaseVery sensitiveDestructiveDecompositionAble to identify wide range ofSlowInductively Coupled-contaminantsExpensiveMass SpectrometryComplex(VPD ICP-MS)Cannot imageOnly works on bare siliconSecondary Ion MassVery sensitiveExpensiveSpectroscopy (SIMS)Detects a wide range ofSlowcontaminantsDestructiveSub-surface detection
Certain types of contamination and residues can be detected with optical or analytical metrology tools. Optical inspection systems are often fast enough for in-line inspection of production wafers. However, these tools are not well suited to detecting small amounts of non-optically-visible contamination, film non-uniformities, chemical contamination, or variations in chemistry. Analytical tools, such as TXRF or TOF-SIMS, provide a wealth of information on wafer surface chemistry, but are expensive, extremely slow, destructive, or don't provide information on the whole wafer surface. Effectively detecting and controlling contamination during wafer processing requires a fast, in-line, and nondestructive method of detecting minute chemical changes on the wafer surface. The aforementioned chemical analytical methods cannot operate under such demanding in-line materials processing conditions. This capability would lead to cost savings by minimizing the time required to detect contamination, thereby reducing the number of wafers affected and the associated costs of scrap or reduced device reliability.
In general, existing in-line wafer inspection tools operate at production speeds and generate images of the wafer surface that are processed to identify and locate defects. However, these techniques are, as mentioned above, very limited in their ability to detect chemical contamination. Laser backscattering systems are limited to detecting particles down to sub-micron sizes, and optical microscopy systems can only detect chemical contamination that results in a visible stain or residue. Both techniques lack the ability to identify or classify the chemical composition of the particle or contamination. Off-line laboratory techniques are used to qualify the cleanliness of new processes and equipment, or to analyze defects detected by in-line equipment or as part of failure analysis.
Another system that has been investigated is the use of Contact Potential Difference (CPD) imaging. CPD refers to the electrical contact between two different metals and the electrical field that develops as a result of the differences in their respective maximum electronic energy level, i.e. their respective Fermi energies. When two metals are placed in contact, the Fermi energies of each will equilibrate by the flow of electrons from the metal with the lower Fermi energy to that of the higher. “Vibrating CPD sensor” refers to the vibration of one metal relative to the other in a parallel plate capacitor system. The vibration induces changes in the capacitance with time, and therefore a signal related with the surface profile. A CPD signal can also be generated by the translation of one surface past a reference sample through the use of a non-vibrating contact potential difference (nvCPD) sensor(s). This translation makes high speed scanning possible.
However, even these nvCPD sensors can themselves present certain difficulties. At a microscopic level, the surfaces of wafers are not flat due to wafer thickness variation, materials on the surface, “bowing”, and other factors. In order to scan the wafer at a close but safe distance (i.e., close to the surface to promote good signal strength but far enough away to minimize any possibility of impacting the wafer surface), an appropriate sensor height must be calculated and set. Thus, the height of the sensor above the wafer surface must be measured and controlled to produce repeatable results. Furthermore, height control is also necessary to minimize the sensor height to improve resolution and signal strength. However, height is difficult to control and measure, as is the appropriate height for measurements on each specific wafer.
It is possible to use one of many commercially available height sensors to control the height of the nvCPD sensor above the wafer surface. This requires the expense of an additional sensor, and the added complexity of a calibration routine to determine the position of the nvCPD sensor tip relative to measurements made by the separate height sensor.
A related problem is the difficulty in establishing a point of reference for all distance measurements, including height, related to an nvCPD scan. A reference point is needed to produce useful measurement data for image production.
In some sensor systems, such as nvCPD sensors, it is necessary to separate the sharp peak signal from the other two components of the signal (low frequency signal and induced noise signals) to locate and measure the contaminated areas of a wafer. This is challenging because the sharp peak signal behaves like noise, i.e., it consists of sharp peaks that alternate their polarity in high frequency mode. Because of this, conventional high frequency filters based only on the frequency domain do not work, as they would degrade the sharp peak signal significantly along with the noise.
In addition, an nvCPD signal is generally delayed in time, which impacts on the quality of the nvCPD signal/image. As the sampling time increases, the time delay becomes larger. The time delay may be explained by the equivalent RC circuit modeling the electrical signal path from the probe tip to the output of the A/D converter through the amplifier, the data acquisition board and the connecting lines between them. The equivalent capacitance is mixed with the capacitance between the probe and the wafer surface, the parasitic capacitance of the connecting lines, the internal capacitance of the amplifier, and other known conventional effects. The result is that minute feature signals are less detectable, and the signal magnitude and thus the signal-to-noise ratio are smaller.
Furthermore, topographical features of a wafer often produce a weak signal in comparison to the signals from chemical features. As the usefulness of topographical versus chemical features often varies depending on the particular circumstances of an imaging application, there exists a need to be able to amplify the signal indicating topographical features or to separate, superimpose, reduce or remove signal indicating chemical features.
Also, many different types of imaging systems currently rely on a chuck to spin a sample material, such a semiconductor wafer, relative to the probe apparatus. These current designs scan the sample surface at a constant rotational speed. The probe then scans the wafer by taking circumferential tracks of data at a constant sampling rate. Due to constant rotational speed and a constant sampling rate, it is apparent that the angular separation of an individual sample will be constant over the wafer surface. However, the actual physical spacing of the data in Cartesian coordinates varies with the radius of the track being scanned. In effect the data becomes denser as the radius decreases. In addition the amount of current generated in the sensor is a linear relationship with the relative speed of the probe to sample. The actual relative speed of the sample to the probe is then related to the radius of the track of data being collected so it is not a constant when the sample is scanned at a constant rotational speed. This results in signals of larger value on the outer radius of the sample and lowers the signal towards the center of the sample which results in higher signal to noise ratio than if data density were maintained at a substantially constant level.
In addition, a need exists to increase the overall accuracy, speed, and efficiency of current inspection systems. Current systems do not meet the increasing demand from the industry to provide a method of testing a wider variety of products in a more efficient and faster manner.
In addition to a need for a system to increase the overall speed and efficiency of current inspection systems, there exists a need to provide a system which can both detect point defects, i.e. those that occur on some region of a wafer, and those whole wafer defects, i.e. those which occur uniformly across the wafer.
A need also exists for an improved scanning device which addresses the shortcomings of nvCPD sensors. Detection systems based on nvCPD systems, such as some embodiments described in the parent applications, have difficulty detecting uniform defects which occur across the entire wafer. In addition, nvCPD systems, while excellent at providing relative CPD data, are unable to determine the absolute CPD of a point on the surface. Such information is useful in both identifying material on the surface of a sample as well as in defect determination and identification.
Contamination of a whole wafer is a relatively common problem. One common defect of this type is where a chemical or elemental layer is uniformly deposited on the wafer, either where it was not intended to be or where an additional layer than was intended is applied. Conversely, wafers are produced which lack entire layers that were intended to be applied, such as where an oxide layer is not deposited as it should have been. Both of these, and particularly the last, provide distinct and difficult challenges for the inline inspection of wafers. Currently, offline, destructive techniques as described above are needed to detect this category of defect.
For example, manufacturers apply very thin (on the order of 30 nanometers) organo-silane films to wafer surfaces to function as lubrication. The minute amount of material is undetectable by optical inspection tools. The industry must utilize difficult and time-consuming tests to specifically address this issue. Thus, there exists a need to determine whether the wafer has a layer deposited thereon, and if so, to determine if the layer is of uniform thickness.
Another application where current inspection devices fail involves the removal of a thin film from a wafer or other electronic device surface. For example, it is sometimes desirable to remove a photoresist layer from a wafer. Current systems cannot efficiently determine whether the thin layer of photoresist has been removed. A need exists for a technique which can detect the presence or absence of a thin layer, while still providing information regarding surface defects.
Yet another industrial application which is in want of an improved inspection system involves the use of “witness wafers”. Witness wafers are used to monitor the air quality in a semiconductor manufacturing clean room. The witness wafers are left exposed in the environment for a period of time, during which the volatile organic compounds present in the air are deposited on the wafer surface. Analytical techniques used to evaluate the witness wafer are slow and only able to spot check the wafer. An inspection method which is able to defect check the entire surface as well as to check for deposited layers of volatile organics is needed.
In the manufacturing situation, one common case of uniform contamination involves the use of a contaminated solution or deposition chamber. This may occur, for example, if the wafer is placed in a contaminated solution (for example during cleaning or plating) or in a contaminated deposition chamber (such as a Physical Vapor Deposition (PVD) or Chemical Vapor Deposition (CVD) system). In this case the contamination may be uniform across the entire wafer. Thus, processes which detect changes on the surface (e.g. nvCPD and optical) would not detect these defects.
A critical need therefore exists for a fast, inexpensive, and effective means of detecting, locating, and classifying relatively small quantities of chemical content or features and physical features on samples (and any surface which dictates ultimate performance of an electronic or chemical device), including the presence or absence of completely or nearly complete missing or additional layers on a wafer or other material. There is also a need for a system which minimizes cost and complexity of the sensor control mechanisms, such as height control. Furthermore, there is a need for methods and systems that have improved signal processing.